1. Field of the Invention
The present disclosure is generally related to communication systems, and, more particularly, is related to wireless communication systems and methods.
2. Related Art
Wireless communication systems are widely deployed to provide various types of communication, such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division multiplex (OFDM), or some other multiplexing techniques. OFDM systems may provide high performance for some channel environments. In OFDM systems, the high-speed data signals are divided into tens or hundreds of lower speed signals that are transmitted in parallel over respective frequencies within a radio frequency (RF) signal that are known as subcarrier frequencies (“subcarriers”). The frequency spectra of the subcarriers overlap so that the spacing between them is minimized. The subcarriers are also orthogonal to each other so that they are statistically independent and do not create crosstalk or otherwise interfere with each other. Further, each block of data is mapped into each subcarrier as frequency domain symbols. The symbol duration is much longer than the length of the channel impulse response so that inter-symbol interference is avoided by inserting a cyclic prefix for each OFDM symbol. Thus, OFDM is much less susceptible to data loss caused by multipath fading than other known techniques for data transmission. Also, the coding of data onto the OFDM subcarriers takes advantage of frequency diversity to mitigate loss from frequency-selective fading (e.g., if forward error correction (FEC) is applied).
In a terrestrial communication system (e.g., a cellular system, a broadcast system, a multi-channel multi-point distribution system (MMDS), among others), a RF modulated signal from a transmitter unit may reach a receiver unit via a number of transmission paths. The characteristics of the transmission paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used for data transmission. If the transmission paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A multiple-input multiple-output (MIMO) communication system employs multiple transmit antennas and multiple receive antennas for data transmission. A MIMO channel formed by the transmit and receive antennas may be decomposed into independent channels. Each of the independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension.
When using MIMO systems with OFDM multiplexing, the frequency diversity provides an added dimension to a MIMO system that can provide improved performance, but also increases the complexity of a system. For example, signals received at a receiver may be distorted versions of the transmitted signals because of transmitter/receiver imperfections and/or environmental effects that can change the amplitude and phase of the signals, resulting in an increase in the bit error rate at the receiver. In an IEEE 802.11 (herein, “802.11”) compliant system (e.g., 802.11a, 802.11g), for example, a receiver may be upconverting a received signal to 2 primary bands (2.4 giga-Hertz (GHZ) or 5.8 GHz), and often there is phase noise (e.g., root mean square (RMS) phase jitter or jitter phase noise) on the voltage controlled oscillators corresponding to upconversion at the transmitter and downconversion at the receiver. In a MIMO-OFDM system, such jitter phase noise is complex.
To remedy these and other signal distortions in single transmit antenna, single receiver antenna (SISO) systems, transmitters may send known signals, such as pilot tone signals, in a preamble portion or data portion (e.g., each transmitter transmits frequency domain coded symbols in which a respective portion of the frequency sub-channels is allocated for the transmission of known pilot symbols values and the remainder is allocated for data values) of a frame being sent to use at the receiver for compensating for distortions in the received signal. For instance, a communication system may transmit a total of 52 subcarriers with each OFDM symbol, with 48 subcarriers carrying data and 4 subcarriers carrying pilot tones. The receiver recovers these pilot tones and uses the same to compute the degree of jitter phase noise on the received signal and then corrects accordingly.
In MIMO-OFDM systems with multiple upconversions and downconversions and cross coupling among multiple transmit and receive antennas, correction of phase jitter and other noise can be complicated, sometimes resulting in destructive interference of signal information at a receiver device.